Unusual Arctic warmth as north hemisphere shivers

WASHINGTON Tue Jan 12, 2010 11:02am EST

Broken Arctic sea ice as seen from a window in from a U.S. Coast Guard C130 flight over the Arctic Ocean September 30, 2009. REUTERS/Yereth Rosen

Broken Arctic sea ice as seen from a window in from a U.S. Coast Guard C130 flight over the Arctic Ocean September 30, 2009.

Credit: Reuters/Yereth Rosen

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WASHINGTON (Reuters) - While much of the Northern Hemisphere has shivered in a cold snap in recent weeks, temperatures in the Arctic soared to unusually high levels, U.S. scientists reported.

This strange atmospheric pattern is caused by natural variability and not by rising levels of greenhouse gases. However, it could affect Arctic ice which in turn may impact global warming, said Mark Serreze, director of the U.S. National Snow and Ice Data Center in Colorado.

"It's very warm over the Arctic, with air temperatures locally at 10 to 15 degrees F (5.6 to 8.4 degrees C) warmer than they should be in certain areas," Serreze said in a telephone interview on Monday.

This contrasts with record or near-record cold over much of the eastern United States and Canada, Europe and Asia for the last two weeks of December and the first days of January, the data center reported.

It's due to a large area of high pressure over the Arctic, and a big area of low pressure at the mid-latitudes, where much of the Northern Hemisphere's population is concentrated.

Usually these areas of differing air pressure would shift and mix in a phenomenon known as the Arctic oscillation. Instead, they've remained stationary in what scientists term a negative phase of the oscillation. A positive phase would have low pressure over the Arctic and high pressure over the mid-latitudes.

Serreze said that as of December, the oscillation was in the most extreme negative phase seen since modern record-keeping began in 1950.

"Normally the circulation of the atmosphere would mix these two (areas of varying air pressure) together, and it's not doing a very good job of that right now, so we have these blobs of warm air over the Arctic and these blobs of cold air over the mid-latitudes, just sitting there," he said.

The blobs appear to be starting to shift, a sign that the negative phase is weakening.

The extent of Arctic sea ice at the end of December remained below normal, some 350,000 square miles (920,000 square kilometers) below the average for December from 1979-2000. But it was above the 2006 record low for the month.

The low level of Arctic ice could accelerate climate warming because there is less light-colored ice to reflect sunlight and more dark-colored ocean water to absorb it.

However, the Arctic oscillation could have an opposite effect. With the blobs of pressure in place, the ice that now covers the Arctic is less likely to be moved south to melt. This could help build up the older, hardier Arctic ice, which means more ice to reflect the sun's warming rays.

(Editing by Alan Elsner)

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